What is baptism for the dead?

In his first letter to the Corinthians Paul writes

Otherwise, why do they do it, those who are being baptized on behalf of the dead? If the dead are not raised at all, why indeed are they being baptized on behalf of them? (1Co 15:29)

This is a much debated passage. What does Paul mean here? And is baptism for the dead a valid practice?

I am not aware of any translation issues here. It appears that Paul is referring to a practice where some people are being baptised on behalf of other people who have already died.

There are several things worth noting about this phrase.

1. Paul uses the pronoun "they"
2. The Greek for "baptism" means "dipped" or "immersed". Though baptism as a ceremony is probably meant, immersion could have another meaning in the passage.
3. Paul is using this practice to illustrate his point.

On point 3, Jesus, Paul, and others may refer to a concept to illustrate a truth without approving of the concept.

Prior to these verses Paul summarises they gospel he taught them (1Co 15:1-11). Within this summary he mentions that Christ was raised on the third day then appeared to many. Following this Paul addresses the issue of resurrection, specifically that some of the Corinthians were denying the resurrection.

Now if Christ is preached as raised up from the dead, how do some among you say that there is no resurrection of the dead? (1Co 15:12)

Paul then argues for the truth of the resurrection from the dead. Firstly by arguing the consequences if the resurrection is not true; then affirming the truth of the resurrection and what that means. It is following Paul's defense of the resurrection that Paul makes this statement of baptism on behalf of the dead.

It is possible that Paul here is showing up the inconsistency of being baptised on behalf of the dead yet denying the resurrection.

Now if Christ is preached as raised up from the dead, how do some among you say that there is no resurrection of the dead? ...Otherwise, why do they do it, those who are being baptized on behalf of the dead? If the dead are not raised at all, why indeed are they being baptized on behalf of them? (1Co 15:20,29)

If this is the case, then Paul is showing the inconsistency of those denying the resurrection by illustrating that they are baptised on behalf of the dead, a practice that logically implies that the resurrection is true. If there is no resurrection from the dead then it is pointless to be baptised on behalf of an already dead person.

Further, if the people who deny the resurrection whom Paul is addressing are the same people who are practicing baptism on behalf of the dead, they are not people whom we should emulate.